This invention relates to waterless planographic printing. More particularly it relates to novel photosensitive sheet constructions suitable for use in waterless planographic printing which do not require liquid development.
In planographic printing, as the name implies, both image and nonimage areas lie substantially in the same plane, i.e. the ink-receptive image areas are not substantially raised from the surface of the printing plate. The nonimage areas are made ink-repellent so that upon ink application to the plate surface, only image areas accept the ink for subsequent transfer to a sheet material.
Conventional lithography, the best known form of planography, generally requires a dampening of the printing plate with an aqueous fountain solution to effectively wet the nonimage plate areas after which ink is rolled over the plate. The oily ink selectively wets the oleophilic image areas but is repelled from the dampened nonimage areas.
To overcome difficulties inherent with conventional lithography, planographic printing plates not requiring dampening have recently been developed. These plates require only an inking system to be operative, and inherently contain ink repellent nonimage areas. This relatively new concept has come to be known by the term driography, and plates useful therein have been termed driographic plates. Such a printing plate is disclosed in U.S. Pat. No. 3,511,178.
Driographic plates generally operate on the principle that the nonimage or background surface areas have sufficiently low adhesion to driographic printing inks that ink applied to the plate by an inking roller will not split away or transfer from the roller to the plate in such areas. A surface exhibiting such characteristics has been termed "adhesive". An analysis of driographic printing processes and ink requirements therefor is presented in Pira's Process Product Survey, Driography, Surry England (1972).
At present, one of the most common techniques for providing ink-receptive imagewise areas on driographic plates is by selective removal of the adhesive coating. For example, in the aforementioned U.S. Pat. No. 3,511,178, a material which is adhesive, e.g. a curable silicone elastomer, is coated over a light-sensitive diazo resin layer on a suitable substrate. Liquid development of the imaged plate removes the unexposed diazo resin and silicone elastomer overcoat in an imagewise fashion. Another imaging technique is to overcoat an adhesive material with an ink-receptive composition which can be selectively removed in an imagewise fashion in unexposed areas with a liquid developer. In this instance it is extremely difficult to obtain a satisfactory bond between the adhesive material and the exposed overlayer.
In the aforementioned driographic concepts a development step is necessary to provide a press-ready plate. This step adds a time delay and introduces additional processing equipment and supplies in the production of a driographic plate. Additionally, skill and training is required for plate preparation.